In producing printed wiring boards, small holes are formed in blank boards for the printed wiring boards by placing the blank boards in the form of superposed layers on a backup board, disposing an entry board on the upper surface of the uppermost blank board, and forming a through hole in the entry board from above and also a small hole in all the blank boards at a time using a drill in this state as is known.
Conventionally used as the entry board is an aluminum board, e.g., an aluminum board of AA1100-H18 material, chiefly for the purpose of preventing defacement and reducing formation of burrs when holes are drilled in the blank board for the printed wiring board. With electronic components mounted on printed wiring boards with a higher density in recent years, it is required to form small holes of up to 0.3 mm in diameter. This gives rise to the problem that use of aluminum boards as entry boards permits drill bits to skid on the board surface, frequently breaking drill bits. Because of the frequent breaks in drill bits, it is impossible to superpose an increased number of blank boards for drilling to provide printed wiring boards, hence the problem of a poor efficiency. Further since the drill bits skid on the board surface, there is the problem that small holes are not formed in position accurately. Another problem is encountered in that the inner peripheral wall defining the small hole formed becomes rough-surfaced.
Accordingly, it appears feasible to use as an entry board for use in drilling small holes an aluminum base board having a lubricant layer formed on at least one surface thereof by affixing a lubricant sheet thereto in order to overcome these problems and to prevent breaks in drill bits, eliminate roughness from the inner peripheral surfaces defining small holes and ensure improved accuracy in the position of the small holes to be formed in the blank board. Available as the lubricant sheet is, for example, a sheet having a thickness of 0.1 to 3 mm and prepared from a mixture of 20 to 90 wt. % of a polyethylene glycol having an average molecular weight of at least 10000 and 80 to 10 wt. % of a water-soluble lubricant as disclosed in JP-A No. 4-92494. However, the lubricant sheet exhibits poor adhesion to the aluminum base board, becomes locally separated from the base board and becomes uneven in its thickness, or irregularities occur in the surface of the entry board facing the blank board during drilling to cause a break in the drill or to impair the accuracy in the position of the small hole. Furthermore, the lubricant sheet is likely to crack or become uneven in thickness, or irregularities will occur in the surface of the entry board facing the blank board during drilling, consequently causing a break in the drill or impairing the accuracy in the position of the small hole.
An object of the present invention is to overcome the foregoing problems and to provide an entry board for use in drilling small holes wherein a lubricant layer is excellent in adhesion to a base board thereof and can be prevented from cracking.